why is my girlfriend lying to me about her past?

my girlfriend and i have been together for over a year now and im having trouble trusting what she tells me about her past. a few months ago we randomly discussed our past and she told me that she had slept with 4 guys and kissed 4 others. we were drunk 2 nights after and got onto the same converstaion. this time she told me she had slept with 5 people and she cant remember how many people shes kissed. over the last few months ive also found more stuff out about her past that she didnt tell me...like having another relationship and spending the night with her ex just before we were officially together. im having a really hard time sleeping and generally being happy because its bugged me so much that she would lie to me. i feel like i should break up with her because in my eyes it is not something that you lie about.
am i just being stupid or should i wake up and stop putting up with all of the lies?? please please please help. thanks
ollie x